How do you know how much you are bench pressing?

Hey Everyone,
I am new to the world of weightlifting and had a quick question. And forgive me if this is a dumb question, but I can't tell how much I am bench pressing.

If I place 1 45 pound plate on either side of the bar am I just lifting 45 pounds, Or am I lifting 45 pounds plus the weight of the bar and how much to the bars weigh? The equipment at my gym is all StarTrac brand, and they have the StarTrac Max Rack systems for benching, squats, etc.